1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an array substrate of a liquid crystal display device for providing an on-common type storage capacitor by 4-mask processes, and a manufacturing method thereof.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) device includes two transparent substrates, with a gap between them, a liquid crystal material, being optically anisotropic and injected between the two transparent substrates, and a driving element for applying voltages to the liquid crystal material.
Nowadays, such LCD devices are used for the display means of computers or the like, and their display areas have been growing. The driving of a large LCD device is achieved by employing an active matrix-type array structure having several tens of thousands of pixels with data lines and gate lines passing the periphery of each of the pixels, and a thin film transistor as the driving element placed at each point where the data line and the gate line cross.
In such an active matrix-typed LCD device, it is necessary to maintain the signal voltage input through the data line for a period of time until the input of the next signal voltage is provided to the data line so as to ensure the uniformity of the image. To achieve this, a storage capacitor is formed in parallel with a liquid crystal cell.
The storage capacitor formed in the LCD device is classified into either an on-common type or an on-gate type according to how the electrode is used for charging.
The on-gate type storage capacitor uses a part of an (n−1)th gate line as a storage electrode of an nth pixel. It has advantages of a low decrease of aperture ratio, low incidence of point defects occurring in the normally white mode (NW mode), and a good production yield, but it has a disadvantage of long scanning signal time.
The on-common type storage capacitor employs an additional separate charge electrode. It has advantages such as short scanning signal time, but it has disadvantages of high decrease of the aperture ratio, a significant rate of point defects occurring in the normally white mode (NW mode), and a low production yield.
Now, referring to FIG. 1, a simple description of the on-common type storage capacitor will be described.
FIG. 1 is a schematic representation of the related art array substrate of an LCD device having the on-common type storage capacitor formed thereon.
In reference to FIG. 1, the array substrate of the LCD device having the on-common type storage capacitor formed thereon is configured to include a plurality of gate lines 109, 119 intersecting a plurality of data lines 110, 120 on an insulating substrate as a lower substrate. At the intersection of a data line (for example, 110) and a gate line (for example, 119), there is formed a thin film transistor (TFT) which is composed of a source electrode 111 and a drain electrode 112 in the same circuit layer as the data line 110, a gate electrode 114 in the same circuit layer as the gate line 119, and a semiconductor layer 113.
Further, a pixel electrode 115 is formed to be connected to the drain electrode 112 and spaced from the gate line 119 and the data line 110, and a lower storage electrode 116 is located in parallel with the gate line 119, stretching across the pixel electrode 115.
The on-common type storage capacitor structured as above stores electric charges between the pixel electrode 115 acting as the upper storage electrode, and the lower storage electrode 116 formed of the same material as the gate electrode 114. The capacitance of the storage capacitor structured as above is determined by the formula:
      C    =          ɛ      ⁢              A        d              ,where C is capacitance, ∈ is dielectric constant, A is the area of the electrode, and d is the separation between the electrodes.
The capacitance of the storage capacitor is required to be large enough to ensure the uniformity of the image displayed on the LCD device.
Another method of accomplishing this function is a capacitor with a separate upper storage electrode formed under the pixel electrode, and a reduced separation distance d between the electrodes is so as to increase the capacitance as shown in FIGS. 2 and 3.
FIG. 2 is a schematic representation of a related art improved array substrate of an LCD device having an on-common type storage capacitor formed thereon, and FIG. 3 is a detailed sectional view of the I portion of FIG. 2.
In reference to FIGS. 2 and 3, the basic structure of the array substrate having the improved on-common type storage capacitor formed thereon is similar to the structure of the related art array substrate of FIG. 1, but with the difference of the changed structure of upper storage electrode 217.
Accordingly, the like elements in the FIG. 1 will be referred to as like numerals, and the different elements from those of FIG. 1 will be described herein after.
As shown in FIGS. 2 and 3, the improved on-common type array substrate includes the upper storage electrode 217 having a predetermined area formed on the same layer as a data line 110 and under a pixel electrode 115 using the same material as the data line 110.
A through hole region 305 having through holes exists in a part of a protecting layer 303 covering the upper storage electrode 217, and the pixel electrode 115 and the upper storage electrode 217 are electrically connected through the through hole region 305.
The on-common type storage capacitor structured as above, stores electric charges between the upper storage electrode 217 formed as the same material as the data line 110, and a lower storage electrode 116 formed as the same material as a gate electrode 114.
In comparison to the on-common type storage capacitor in FIG. 1, because the separation of the two electrodes of the storage capacitor is reduced, a greater capacitance can be achieved.
Such on-common type storage capacitors, thin film transistors, pixel electrodes, and the like are formed through five photo processes. That is, the manufacturing process uses five masks.
However, each photo process involves complicated processing steps, and each additional process step results in an increase in process failures. Therefore, increasing the number of photo processes involved increases the occurrence of failures, thereby deteriorating the production yield of substrates.